1. Field of the Invention
The present invention relates to an optical transmitter module used primarily in an optical transmission system or in an optical switching system (both referred to as an optical communication system) and more particularly to an optical connecting technology used for connecting a light emitting or receiving element with optical fibers in the optical transmission module, or a light emitting or receiving element with an optical circuit, as well as an optical circuit with optical fibers.
2. Description of the Prior Art
Construction of communication lines using optical fibers is projected not only among office buildings of a variety of industries but also among condominiums and residences, as the optical technology of communication develops. One of problems concerned is the cost of the optical transmission system, in particular the rate of lowering the cost of optical modules that reach to the subscribers. In the prior art, an optical lens has been usually inserted between an optical element such as semiconductor laser and an optical fiber or an optical waveguide, in order to increase the optical coupling efficiency. This approach of providing a lens between an optical element and a waveguide has many drawbacks, for lowering the cost of optical modules for subscribers, such as increased number of parts, alignment required between three independent members, and consequently a complex work to mate them. In order to solve these problems, for example, as disclosed in Japanese Unexamined Patent Publication No. H5-249331 and Japanese Unexamined Patent Publication No. 2000-214340, a semiconductor laser integrated with a beam spot size converter has been devised and developed to make practicable.
To explain this type of semiconductor laser having a converter of spot radius of beams, a way for determining the coupling efficiency of light beam will be described now by referring to a schematic diagram of binding between a semiconductor laser and a light waveguide in FIG. 10. Now the light beam is assumed to be a Gaussian beam. The coupling between two 0th order Gaussian beams will be considered here, as in an optical communication, the waveguide mode of beam in many cases is in single mode. The coupling efficiency η may be given by the following equation:                     η        =                  κ          ⁢                                           ⁢          exp          ⁢                      {                                          -                κ                            ⁢                                                x                  2                                2                            ⁢                              (                                                      1                                          W                      1                      2                                                        +                                      1                                          W                      2                      2                                                                      )                                      }                                              [                  eq          .                                           ⁢          1                ]            
where beam spot sizes (the radius that the amplitude of a Gaussian beam becomes 1/e of the median value) in each of beam waists of a semiconductor laser 23 and a waveguide 22 (where the radius of curvature of the wavefront of Gaussian beams is infinite) are W1 and W2 respectively, Z is the distance between these beam waists, X is the offset in the direction perpendicular to the optical axis, λ is the wavelength of beam to be carried. In the equation above, κ may be given by:                     κ        =                  4                                                    (                                                                            W                      1                                                              W                      2                                                        +                                                            w                      2                                                              w                      1                                                                      )                            2                        +                                          (                                                      λ                    ⁢                                                                                   ⁢                    z                                                        π                    ⁢                                                                                   ⁢                                          W                      1                                        ⁢                                          W                      2                                                                      )                            2                                                          [                  eq          .                                           ⁢          2                ]            
From the above equations, the tolerance with respect to the coupling efficiency and axial offset will be improved when (1) W1=W2, and (2) these values becomes as large as possible.
The beam spot size W1 of a conventional semiconductor laser 23, in other words the incident beam spot size W1 is much smaller when compared with the beam spot size W2 of the optical fiber or waveguide 22 so that W1 will not be equal to W2. It can be concluded that the coupling efficiency will be good.
The semiconductor laser integrated with the attachment of a beam spot size converter has an effect to approaching W1 to W2 by enlarging W1 in order to improve the coupling efficiency and tolerance as a result.
However, a semiconductor laser integrated with the attachment of a beam spot size converter is constructed by using selective crystalline growth technology in order to tapering the film thickness at the emitting end of a core. The integration of beam spot size converter may affect the optimum design of the laser used, or the sensitivity of laser characteristics with respect to the construction error. As a consequence the conventional laser system has an insufficient yield, which pushed up the price of laser elements and finally a considerable reduction of cost of the optical modules is not yet achieved.
The enlargement of beam spot size by tapering by the selective crystalline growth method is limited to, at present, approximately 10 degrees when expressed by the angle of divergence of far field image when approximating a Gaussian beam. When taking into consideration the fact that the divergence of optical fibers is in the order of approximately 5 degrees, the difference between these two is still large. Even when using a semiconductor laser with a beam spot size expander converter, an innovative technology of a novel coupling scheme in combination therewith is needed in order to achieve a higher optical coupling efficiency along with ease of construction.
There is, needless to say, other approaches to solve the problem by the Prior Art. One solution is to incorporate an optical lens between a semiconductor laser and a waveguide. However, this solution may result in an increase of the number of parts and a complex method of production so that the achievement of lowering cost of optical modules will be difficult.